Recently we had a question about how to populate multivalued virtual attributes in ARS within same QC workflow.
In this specific scenario synchronization step 1 would create the user account, writing data to the multivalued virtual attribute.
Then synchronization step 2 would update (append) data to that multivalued virtual attribute from a different source system.
In this case you need to select rule updating this attribute, press “More” button and choose “Options…”.
You will see window like on screenshot below and you need to select “Append source values to target attribute values”.